PROJECT SUMMARY Systemic lupus erythematosus (SLE) is a disorder of the immune system that is characterized by autoantibody production, hyperactive B cell antigen receptor (BCR) signaling and B cell hyperresponsiveness; however, the mechanisms that regulate aberrant B cell function are poorly understood. Investigation of proteins and processes that drive BCR signaling, B cell hyperactivation and autoantibody production in SLE should offer mechanistic insights and lead to development of more effective therapies. B cell activation is kept under check by a potent inhibitory pathway, a key component of which is the Src family kinase Lyn. Human GWAS studies have shown a strong association between lower expression of Lyn and incidence of SLE. Mice with genetic deletion of Lyn lose peripheral tolerance and display all the symptoms associated with human SLE, including exacerbated BCR signaling, B cell hyperactivation, high levels of serum autoantibodies, and glomerulonephritis. Therefore, Lyn-/- mice represent a clinically relevant model to investigate the molecular regulation of B cell autoimmunity in SLE. We have previously reported that the membrane-cytoskeleton linker protein Ezrin regulates multiple facets of B cell function by undergoing dynamic phosphorylation- dephosphorylation. Interestingly, we observed that ezrin is hyperphosphorylated in Lyn-/- B cells, and that conditional deletion of ezrin in B cells of Lyn-/- mice leads to a significant decrease in B cell activation, differentiation, autoantibody production and immune complex deposition in the kidneys. Our data suggest that ezrin is an important mediator of B cell hyperactivation in the absence of Lyn, and thus a potential molecular target. We hypothesize that ezrin facilitates B cell pathogenesis in Lyn-/- mice by promoting molecular and cellular processes that rely on membrane-cytoskeletal reorganization. Our specific aims are, (1) to investigate the impact of ezrin deletion on immunopathology in vivo, (2) to determine the effect of ezrin deletion on B cell differentiation in vivo, and (3) to examine the effect of ezrin deletion on BCR organization and B cell activation in vitro. We will employ genetic, immunological, proteomic, molecular biology, and high-resolution live-cell imaging techniques to accomplish our aims. We expect that the results of these studies will identify the pathological features of SLE that are altered by loss of ezrin in Lyn-/- mice at different stages of disease progression, ascertain if ezrin promotes autoimmunity in Lyn-/- mice by enhancing germinal center B cell response, and reveal the spatial and molecular mechanisms employed by ezrin to facilitate hyperactive B cell responses in the absence of Lyn. Significance & Impact: We anticipate that the innovative mouse models and experimental analyses proposed here will establish ezrin-dependent membrane-cytoskeletal remodeling as a novel mode of regulating B cell autoimmunity arising from the deficiency of Lyn. Moreover, our results will inform the design of future studies assessing the role of ezrin in disease pathogenesis in other mouse models of SLE, as well as in SLE patients. Ultimately, our studies should lead to identification of new avenues for therapeutic development in SLE.